In using the isotope fluoro-18-deoxyglucose with positron emission tomography, we found that cerebral glucose utilization does not change with advancing age in healthy males, but changes are found in patients with Alzheimer's disease and Down syndrome. Three new protocols were introduced to evaluate these findings and determine their specificity. Studies are underway in multi-infarct dementia, the second leading cause of dementia; major depressive disorder with and without cognitive impairment; and fragile-x syndrome. The role of the dopaminergic system in normal aging, Alzheimer's disease with and without extrapyramidal signs, and familial inverted chorea will be explored with 6-[18-F]-fluoro-L-Dopa and positron emission tomography (PET).